The present invention relates to a measuring stylus, and also to an X-Y digitizer including such a stylus.
One of the major sources of errors in measuring data by the use of an instrument including a measuring stylus is caused by visual parallax, which is defined as the apparent change in the direction of an object caused by a change in observational position that provides a new line of sight. Measuring errors caused by visual parallax are a function of two major elements, namely: (1) the distance between the measuring mark and the measured media, and (2) the angle of view. Thus, in a given fixed position of a measuring mark with respect to the measured medium, different angles of view produce parallax errors which increase with an increase of the angle of view with respect to a line normal to the measured medium. That is to say, the larger is the distance between the measuring mark and the measured medium, the greater will be the parallax error in any given angle of view with respect to a line normal to the measured medium.
One manner of reducing parallax errors is by the optical projection of the measuring mark on the measured medium. However, when the instrument is to be used with media of different thicknesses, the optical projection system requires that the projected measuring mark be refocussed as necessary each time a different thickness medium is being measured. The optical projection system must therefore be sufficiently sophisticated to permit the operator to refocus the measuring mark for different thickness media. In addition, such a system requires a high degree of expertise by the operator in order to refocus the projected measuring mark so as to avoid parallax errors.